When shaping two pieces of material to be joined at a selected angle, there is always a significant problem in providing an angle on each piece such that the two angles will join to form the selected angle. Cutting pieces of wood with a miter guide on a table saw to form mating surfaces on separate pieces to make a frame, such as a picture frame, is an example.
A common operation on a conventional table saw involves cutting a 45 degree angles on two separate pieces of framing material, and subsequently joining the separate pieces, making a 90 degree corner.
FIG. 1 shows a plan view of a conventional table saw 11 having two parallel grooves 13 and 15 for restraining a miter guide 17 to be moved in the direction of the groove. Miter guide 17 has a rotatably adjustable guide element 20 with a support surface 10 for setting the angle by which a piece to be cut is presented to the cutting element, in this case saw blade 23. Adjustable element 20 is pivoted on bar 18, which is configured to slide in groove 13. A hand-operated clamp nut assembly 19 also attaches to bar 18 at a different point than the pivot. By loosening clamp nut 19, rotating element 20, and tightening the nut, a user can adjust the miter to change the angle of the cut. In this description, the miter is shown set to a nominal 45 degree angle, with a frame piece 21 against surface 10.
By sliding the miter guide along groove 13 while holding frame piece 21 against surface 10, the frame piece is brought to saw blade 23, and a cut is made through the frame piece along dotted line 25, such that angle A.sub.1 equals angle A.sub.2, nominally 45 degrees in this example.
The typical procedure to shape a mating piece for joining, after cutting a first piece with the miter guide in groove 13, is to move the miter guide to groove 15, and readjust it, to cut the second piece. Guide 17 is shown in groove 15 in FIG. 1 by dotted outline. In the dotted outline the miter is shown readjusted to provide an angle of nominally 45 degrees (A.sub.3), such that when the miter is moved along groove 15 the saw blade cuts through frame piece 27 along dotted line 29, providing angle (A.sub.4) equal to angle (A.sub.3).
A problem in the conventional method is in the possibly imprecise setting of the angle of the miter, which may be compounded by resetting the miter for cutting the second piece. The common miter guide is not a precision device, and relies on a visual scale. Accordingly, it is quite difficult to set the angle for the miter precisely.
If, in the operation described, angle A.sub.1, hence angle A.sub.2, is not precisely 45 degrees, or angle A.sub.3, hence angle A.sub.4, is not precisely 45 degrees, or both, then the two pieces prepared cannot be joined to form a "tight" corner of precisely 90 degrees.
One might, as a hedge against resetting the miter as described above, leave the miter on the first side of the saw blade, and cut the second piece by turning it upside down and cutting it just as shown for cutting the first piece. The problem in this instance is that if angle A.sub.1 is, for example, less than 45 degrees, such as 44 degrees, then the angle for the second piece will also be 44 degrees, and the pieces would join at 88 degrees. The errors add. If the first angle is greater than 45 degrees, the second angle will also be greater by the same amount, and the resultant joined angle will be greater than 90 degrees by double the error for the individual pieces.
What is clearly needed is a device for ensuring that the angle cut on a second piece corrects any error in the angle cut on a first piece, to produce the desired total angle for the joint to be formed. For example, in the case of a desired corner of 90 degrees, if the angle cut on the first piece is 44 degrees, the angle cut on the second will be 46 degrees without having to readjust the miter.